1. Field of the Invention
This invention pertains to the field of automatic transmission controls.
2. Description of the Prior Art
Automatic transmissions for motor vehicles generally include multiple planetary gear sets, each including a sun gear, a ring gear surrounding the sun gear, and a set of planet pinions in continual meshing engagement with the ring gear and sun gear and supported for rotation on a carrier. Gear ratio changes are made by disengaging a drive connection among the elements of the gear sets that produce a first speed ratio and connecting other components of the gear sets that produce another gear ratio. Hydraulically actuated friction clutches and brakes are employed to releasably connect the drive connections among the components of the gear sets. The friction elements are engaged by supplying pressurized hydraulic fluid to the friction element; the drive connections are disengaged by either venting the friction elements or reducing the pressure in the friction element to such a low magnitude that the friction element has no capacity to transmit torque from its input to its output.
One-way clutches have been used in automatic transmissions in order to avoid the complexity during a gear ratio change of disengaging one friction element and concurrently engaging another friction element. Depending on whether the gear ratio change is an upshift or downshift, the one-way clutch holds then releases or overruns during the period when the oncoming friction element is being pressurized and engaged to complete the new driving connection among the components of the gear sets that produce the new gear ratio.
If the one-way clutch can be eliminated from an automatic transmission, its cost and weight and size can be reduced. However, removing one-way clutches has the potential to degrade the quality of the gear ratio changes, especially during coast-down gearshifts.
If a standard coast-down shift from the third to first speed ratio is attempted during a synchronous gearshift, i.e., one in which the offgoing friction element and oncoming friction element are engaged and released concurrently, a large output shaft torque disturbance results as the drive wheels of the vehicle are connected to the engine if a one-way clutch is not employed.